1. Field of the Invention
The present invention relates to a sheet guide mechanism for use in a printer, and particularly to a sheet guide mechanism for guiding a sheet passing between a platen and a printing head which is moved laterally as it prints.
2. Description of the Related Art
One type of conventional sheet guide mechanism employs a bail roller. The bail roller of this mechanism can be operated manually or automatically.
A conventional sheet guide mechanism employing an automatically operable bail roller will be described with reference to FIG. 11.
The sheet guide mechanism includes a platen 1 and a paper chute 2 positioned substantially under the platen 1. The paper chute 2 has rotatable pinch rollers 3a and 3b attached thereto in order to press paper against the surface of the platen 1. A cut paper sheet can be fed along the upper surface of the paper chute 2. A continuous sheet can be fed along the upper surface of a pin tractor 4 positioned substantially under the paper chute 2. The platen 1 has a shaft 5 to which a separator 6 and a gear 7 are attached. The gear 7 rotates a gear 9 attached to the separator 6 by way of an idle gear 8. A pull-up roller 10 is coaxially attached to a shaft to which the gear 9 is attached and is rotatable when the platen 1 is rotated. A rib 11 is disposed opposite the pull-up roller 10. A printed sheet is discharged after passing between the pull-up roller 10 and the rib 11. A printing head 12 is disposed opposite the platen 1 and is mounted on a carriage which is slidably attached, via a carriage guide portion 13, to a carriage shaft 14. A ribbon protector 15 is attached to the carriage and disposed between the platen 1 and the printing head 12 for guiding sheet.
The sheet guide mechanism further includes bail arm 16 having a bail roller 17 which is attached to one end thereof. The bail roller 17 is urged toward the platen 1 by a spring 18. The bail arm 16 is turnable about a fulcrum 19. A roller 21 attached to one end of a link 20 contacts the bail arm 16. The link 20 is turnable about a fulcrum 22 and has a gear segment 20a. The gear segment 20a meshes with a gear 24 on the shaft of a motor 23. The bail arm 16, the spring 18, the link 20 and the motor 23 are attached to a side frame, not shown. Several bail rollers 17 are provided along the entire length of the platen 1. A sheet pressure plate 26 is attached to a frame 25 and has a tip end which presses paper against the platen 1. A guide rail 27 is attached to the frame 25 for guiding the movement of the carriage. The operation of the sheet guide mechanism will be described hereinafter. In order to print characters, a cut sheet is inserted from behind between the paper chute 2 and the separator 6. The cut sheet is pinched between the platen 1 and the pinch rollers 3a and 3b and the platen 1 is rotated to feed the cut sheet. A controller (not shown) rotates the motor 23 in the direction of the arrow A when a detector (not shown) detects the cut sheet. The link 20 meshing with the gear 24 is turned counterclockwise about the fulcrum 22 and positioned as shown in broken lines. This turns the bail arm 16 clockwise about the fulcrum 19, whereby the bail roller 17 is moved away from the platen 1.
When the cut sheet is delivered between the platen 1 and the printing head 12, i.e., when the cut sheet reaches the printing position, the controller drives the printing head so that the printing operation starts. The cut sheet is advanced upward by the platen 1 during the printing operation. When the upper end of the cut sheet is positioned where the bail rollers 17 are brought into contact with the platen 1, the controller rotates the motor 23 counterclockwise, i.e., in the direction opposite the arrow A. Accordingly, t he bail arm 16 is turned counterclockwise by way of the link 20 so that the bail roller 17 presses the cut sheet against the platen 1 for assuring the guidance of the cut sheet. The cut sheet is further advanced while the characters are printed on it, and the upper end of the cut sheet enters between the pull-up roller 10 and the rib 11 to be guided again.
If a continuous sheet is to be printed, it is fed from the pin tractor 4 toward the region under the paper chute 2. The continuous paper is further fed and the characters are printed on it in the same manner as the cut sheet.
Another type of conventional sheet guide mechanism employs bail rollers (corresponding to bail rollers 17) which can be manually moved toward or away from the platen 1 by operating a lever. The lever moves the bail rollers toward or away from the platen. It is preferable to start the printing operation after the upper end of the sheet is pressed by the bail rollers.
A further type of conventional sheet guide mechanism has no bail rollers. The printing head is moved to a central portion of the sheet (a so-called centering operation) when lines are changed until the upper end of the sheet enters between the pull-up roller and the rib, thereby facilitating the insertion of the upper end of the sheet. After the upper end of the sheet has entered between the pull-up roller and the rib, the centering operations are discontinued and normal printing is carried out.
However, when this type of conventional sheet guide mechanism is used the upper portion of the sheet is guided only by the ribbon protector. Accordingly, away from the printing head, in the axial direction of the platen, the upper portion of the sheet separates from the platen due to the stiffness of the sheet. In this state, sheet vibration occurs during the printing operation. As a result, high quality printing cannot be achieved.
A sheet guide mechanism of the type having automatically operable bail rollers requires links, a motor for driving the bail roller and electric circuits. This increases the size of the mechanism and results in high cost.
A sheet guide mechanism of the type having manually operable bail rollers requires operation of a lever each time a sheet is inserted between the platen and the paper chute or each time the pin tractor feeds a continuous sheet, thus involving laborious operation.
A sheet guide mechanism of the type having no bail rollers requires a centering operation each time a new line is started until the upper end of the sheet enters between the pull-up roller and the rib, thus lowering the throughput of the printer.